Researchers find that the amount of harmful pollution from such fires may be underestimated by as much as 40 percent.

Nearly half the world’s trash is burned in the open, spewing pollutants into the atmosphere that contribute to climate change and affect human health, according to a new study.

Since such burning is largely unregulated and unreported, emissions of some pollutants have been underestimated by as much as 40 percent, said the researchers, who published their findings in the journal Environmental Science & Technology.

“I was shocked at the numbers,” said Christine Wiedinmyer, an atmospheric scientist at the National Center for Atmospheric Research and the study’s lead author. “They were much larger than I expected, particularly the air pollutants.”

The researchers estimated the amount of carbon dioxide, carbon monoxide, mercury, tiny particulate matter, and other pollutants released by burning trash.

Every year 970 million metric tons of food, paper, plastics, and metals are set aflame at homes, businesses, and dumps—roughly 41 percent of the world’s garbage, according to the study.